This invention relates to apparatus suitable for freezing food.
Nowadays, liquid nitrogen and, to a lesser extent liquid carbon dioxide, are widely used to freeze food. A common method of employing the liquid nitrogen or liquid carbon dioxide to freeze food is to advance the food to be frozen on an endless conveyor through a tunnel into which the liquefied gas is sprayed. Heat exchange takes place between the liquefied gas and the food and also between the food and the cold gas that is formed as a result of the vaporisation of the liquid. Fans are typically employed to promote the heat exchange between the cold gas and the food. Food freezing tunnels are, for example, described in U.K. patent specification 2 076 952 B and U.S. Pat. No. 4,171,625.
It is desirable for a food freezing tunnel to be easy to clean. Thus, access should be readily gained after a food freezing operation to the interior of the tunnel. Accordingly, some tunnels are formed with hinged panels on their sides which can be unlatched and raised or lowered manually to enable cleaning hoses and the like to be inserted therethrough. However, the access afforded to its interior by such a tunnel is limited, and difficulties can sometimes arise in achieving effective latching and sealing. Alternative tunnels have means for lifting the roof away from the trough. Some of these tunnels have seals that are positioned near the bottom thereof and are thus likely to be directly impinged upon by the liquid nitrogen sprays. Other such tunnels employ relatively complex roof constructions that are difficult to fabricate or involve complex lifting arrangements. Moreover, such tunnels can be unsuitable for use in a factory with a low ceiling.
Yet other commercially available freezing tunnels employ a trough member which can be raised and lowered with respect to a fixed roof member. The present invention particularly relates to this kind of tunnel. An example of such a tunnel is described in U.S. Pat. No. 4,171,625. The tunnel employs hydraulic means to raise and lower the trough member. One advantage of this arrangement is that a positive force is applied both to raise and lower the trough member and it is not allowed to fall under gravity. However, typically several hydraulic jacks are required. There is thus a requirement for several hydraulic cylinders and associated pump, reservoir and pipework. The hydraulic lifting system is thus relatively expensive and complex, and in practice can be unreliable. Moreover, there is a risk that the oil or other hydraulic fluid may contaminate the food to be frozen.
Another tunnel of this kind substitutes pneumatic actuators for the hydraulic ones. The use of such pneumatic actuators means that a pneumatic pressure needs to be applied to the actuators throughout the period in which the trough member is in its raised position. Moreover, the pneumatic actuators restrict the height available for downward travel of the trough member and difficulties can arise in operating them in unison with the result that damage can be done to the carcass of the trough member. Furthermore, the pneumatic actuators are single acting in the upward direction. Gravity is used to lower the trough, so delay may be caused by ice deposited between the engaging surfaces of the roof and trough during operation of the tunnel, with the result that the ice has first to melt before the trough can be lowered.
There is thus a need for a tunnel having improved means for moving the trough member relative to the roof member, and the invention aims at meeting this need.